a2 – 3a – 54
In order to factorize the given expression, we find to find two numbers p and q such that:
p + q = -3, pq = -54
Clearly,
6 – 9 = - 3, 6 (-9) = -54
Therefore,
split – 3a as 6a – 9a
Therefore,
a2 – 3a – 54 = a2 + 6a – 9a – 54
= (a - 9) (a + 6)
Therefore,
(a + 7) (a – 10) + 16 = (a – 9) (a + 6)